Organic Chemistry

Section 15.4 Some Chemical Consequences of Aromaticity 599

Similarly, furan and thiophene are stable aromatic compounds. Both the oxygen in
the former and the sulfur in the latter are hybridized and have one lone pair in an
orbital. The second lone pair is in a porbital that overlaps the porbitals of adjacent
carbons, forming a bond. Thus, they are electrons.

Quinoline, indole, imidazole, purine, and pyrimidine are other examples of hetero-
cyclic aromatic compounds. The heterocyclic compounds discussed in this section are
examined in greater detail in Chapter 21.

PROBLEM 5

In what orbitals are the electrons represented as lone pairs when drawing the structures of

quinoline, indole, imidazole, purine, and pyrimidine?

PROBLEM 6

Answer the following questions by examining the electrostatic potential maps on p. 598:

a. Why is the bottom part of the electrostatic potential map of pyrrole blue?

b. Why is the bottom part of the electrostatic potential map of pyridine red?

c. Why is the center of the electrostatic potential map of benzene more red than the

center of the electrostatic potential map of pyridine?

15.4 Some Chemical Consequences of Aromaticity

The of cyclopentadiene is 15, which is extraordinarily acidic for a hydrogen that
is bonded to an hybridized carbon. Ethane, for example, has a of 50.

CH 3 CH 3 +

HHH

CH 3 CH 2 H+

+ H+

cyclopentadiene

ethane ethyl anion

cyclopentadienyl

anion

−

−

pKa = 50

pKa = 15

sp^3 pKa

pKa

NNN

quinoline indole imidazole purine pyrimidine

N

H

N NH

NN

N

N

H

resonance contributors of furan

OO OO O

−−

++++− −

p p

sp^2

sp^2

N H O

orbital structure of pyrrole

these

electrons

are in a

p orbital

these

electrons

are in a

p orbital

orbital structure of furan

these

electrons are

in an sp^2 orbital

perpendicular

to the p orbitals